Achieving stable fixation of two bone segments having an oblique contact surface can be difficult. For example, external rotation injuries of the ankle may often result in a short oblique fracture of the lateral malleolus. Because of the oblique orientation of the fracture line, simple apposition of the bone surfaces is nearly always unstable, since axial loading forces on the bone cause sliding of the two bone surfaces along the oblique fracture line and contribute to potential problems of shortening, non-union, and loss of reduction. For fractures involving the lateral malleolus of the ankle, even as little as 1 millimeter of shortening can lead to debilitating arthritis and ankle instability.
In the case of an oblique fracture of the lateral malleolus, the opposing bone ends of segments or fragments have the tendency to slide in opposite directions and shorten since axial loading of the bone produces shear stresses contributed by the obliquity of the fracture surface. This will be explained with reference to FIG. 1. If a reference point C is considered at the exact center of the fracture, axial forces F on the bone segments 1,2 produce sliding of the segments along the oblique fracture in opposite directions. The displacement of the two bone segments 1,2 in opposite directions causes the axial forces F on either side of the fracture to displace off center, resulting in the production of a force couple or torque T that aggravates the instability. This torque across the fracture site leads to additional displacement of the bone segments 1,2 and shortening of the bone.
A conventional arrangement of treating oblique fractures of the lateral malleolus of the ankle is illustrated in FIG. 2 which comprises inserting two interfragmentary screws or pins 5 across the fracture site. The interfragmentary screws 5 produce simple side to side compression of the oblique fracture surfaces. Because the fracture is oblique and the screws 5 only have a single point of fixation 6 on each side of the fracture surface of fracture 3, the screws 5 rely only on frictional forces across the fracture site to prevent sliding of the oblique surfaces and resultant shortening. Often such interfragmentary screws 5 cannot be placed perpendicular to the plane of the fracture, which results in a contribution of a force component that adds shear to the fracture site. This is similar to the way a wedge will tend to slip out of a vise as the vise is tightened.
Furthermore, since interfragmentary fixation relies on a single point of contact of the screw within each bone fragment 1,2, there is poor resistance to angular displacements and loss of reduction with axial loading of the bone as shown in FIG. 3.
Even if the screws 5 can be placed perpendicular to the fracture, this manner of fixation is not optimal since the bone is often osteoporotic with the result that the threads of the screws strip and lose purchase as compression is applied.
In addition, placing relatively large diameter holes in a small bone fragment can result in further fracture propagation through the screw hole. These factors limit the amount of compression that can be achieved by simple interfragmentary bone screw 5.
Fixation with interfragmentary screws is based entirely on the use of one or two screws. The holding power of the screws is based entirely on the purchase of the screw threads in the thin cortex of the bone fragments 1,2, which is often tenuous. In addition, this type of fixation is extremely weak in resisting external rotational torque T on the ankle, which can also lead to failure of fixation and a poor clinical result.
Another traditional means for fixation of oblique fractures is shown in FIG. 4 in which a bone plate 7 is utilized. Such a bone plate is used to statically hold the fragments 1, 2 in position without any compression (so called neutralization plates) or to hold the bone and create compression along the long axis of the bone (so called compression plates). Neutralization plates do not produce compression at the fracture site but instead depend entirely on the purchase of bone screw 8 on either side of the fracture and the strength of the plate. Fixation is along a single axis and in one plane and is relatively weak in resisting torsional loads. In addition, this type of fixation requires bulky plates and a multitude of screws with larger surgical incisions. Neutralization plates do not provide any load across the fracture site, and may result in longer healing times as well.
Compression plates which produce compression along the axis of the bone are effective for stable fixation of simple fractures that are transverse (not oblique) to the long axis of the bone. However, using a compression plate in the context of a long oblique fracture produces shear forces at the fracture site as previously described, resulting in sliding of the bone fragments with shortening and loss of reduction. In addition, since the plate is situated predominantly within a single plane, like neutralization plates, they provide only a single plane of fixation with limited ability to control rotational forces.
Recently, a plate for distal radius fractures has been used in which there is a small tab extending from a distal edge of the plate to act as a gutter to catch an edge of the bone. However, this tab is designed as a positional reference for the plate to the edge of the bone and is positioned at the extreme distal edge of the plate. Because it does not extend deeply, rotation of the plate would not produce any translational force to the bone, but rather would cause the bone to slip out from under the tab. In addition, the extreme distal nature of the position of this tab would cause the distal fragment to rotate off the corner of the plate.
Another technique for fixation of long bone fractures is the use of intramedullary pinning or rodding. Intramedullary fixation is not effective for short oblique fractures as it provides no rotational control and allows shortening of the bone.